This invention relates to liquid crystal devices, and in particular to liquid crystal displays of the "coincidence addressed" type.
Liquid crystal displays generally comprise a pair of oppositely disposed, spaced apart substrates, a liquid crystal material sandwiched between the substrates, and electrodes on the inside of facing surfaces of the substrates. By "coincidence addressed" is meant that the electrodes are generally in the form of a matrix comprising, for example, a first set of a plurality of parallel, spaced apart elongated conductors on one substrate and a second set of a plurality of parallel, spaced apart elongated conductors on the other substrate and extending orthogonal to the conductors of the first set. Thus the various conductors of one set cross or intersect, without touching, the various conductors of the other set. Thus, by applying a voltage to a selected one of the first set of conductors and a voltage to a selected one of the second set of conductors, a portion of the liquid crystal material between the two selected conductors at the intersection thereof is "activated" (i.e., the light transmitting characteristics thereof are changed) by the presence of the coincident voltages appearing across the intersection. By activating various combinations of selected ones of the liquid crystal portions, referred to as display elements, various optical images can be provided.
Coincident addressing display systems are generally known, and a known requirement of such systems is that in order to avoid such problems as unintended activation of elements not intentionally addressed, each element of the matrix should be electrically unidirectional, e.g., a matrix consisting of light emitting diodes. Known liquid crystal materials, however, are electrically symmetrical, i.e., they have the same electrical characteristics for either polarity of voltage applied thereacross. Thus a liquid crystal display matrix requires the addition of circuit elements, e.g., a diode at each element, to provide such directionality. (See: for example, "Studio Sperimentale del Comportamento Di Strati Di Semicondutlore In Contatto Con Strati Di Cristalo Liquido" by P. Maltese and C. M. Ottavi, Alta Frequenza, V. 44, #12, pp. 727-730 (Dec. 1975). Such liquid crystal diodes suffer the disadvantage in that they cannot operate at high speeds, such as that required for operation of T. V. Thus, they are limited to devices displaying information at low speeds. One solution is the use of a transistor at each display element. See U.S. Pat. No. 3,824,003 on the use of a field-effect transistor connected to each display element of the matrix for use in matrix display panel operating at T. V. rates. However, this method requires a multilayered structure.